1. Field of the Invention
The present invention relates generally to a scanning tunneling microscope (STM) and more particularly to an apparatus having a basic function of an STM for determining a surface shape of an electrically conductive sample and also a function of determining the local potential distribution and the state of electrons on the surface of the sample.
2. Description of the Related Art
A scanning tunneling microscope (STM) is known as an apparatus capable of determining the surface shape of an electrically conductive sample with an atomic level resolution.
A pointed tip of an electrically conductive probe is positioned close to the surface of an electrically conductive sample at a distance of about 10 .ANG.. When a bias voltage U.sub.B is applied between the probe and the sample, a tunnel current flows between the probe and the sample. As is represented by the following equation, a tunnel current I.sub.T depends on a distance S between the probe and the sample exponentially: ##EQU1## where B=a numerical coefficient of about 1.025/.ANG. .sqroot.eV, R.sub.T =a tunnel resistance, and .phi.=a tunneling barrier height (.phi.=(.phi.1+.phi.2)/2; .phi.1: a work function of the probe, .phi.2=a work function of the sample). The tunneling barrier height .phi. of a clean metal surface is about 1 to 5 eV. Thus, from equation (1), it is understood that when S varies by 1 .ANG., the tunnel current I.sub.T varies by about one order. In the STM, the probe is moved over the surface of the sample or an xy plane by means of a fine-motion element such as a piezoelectric element. For example, the probe raster-scans the surface of the sample. During the scan, the distance S between the probe and the sample is controlled with precision of less than 1 .ANG., so as to keep a constant tunnel current. Specifically, the probe or the sample is moved perpendicularly to the surface of the sample (i.e. in a z-direction) by means of the fine-motion element such as a piezoelectric element. As a result, the tip of the probe is kept away from the surface of the sample by a predetermined distance, and it traces a curved surface parallel to the surface of the sample. The curved surface reflects the surface shape of the sample. The positions of the tip of the probe on the xy-plane and in the z-axis, which are found from the voltage applied to the piezoelectric element, are recorded, thereby obtaining an atomic-order three-dimensional microscopic image or an STM image showing the surface shape of the sample.
When a sample having constant values of the tunneling barrier height .phi., tunnel resistance R.sub.T and bias voltage U.sub.B (shown in equation (1)), irrespective of locations on the sample, is measured, an obtained STM image reflects the configuration of the sample surface with fidelity. However, when most samples are measured, the surface potential distribution for determining the tunneling barrier height .phi., tunnel resistance R.sub.T and bias voltage U.sub.B varies locally. The tunnel current varies in accordance with the distance between a sample and a probe, microscopic electronic properties of the sample, and local potentials of the sample. Thus, a normal STM image contains data relating to microscopic roughness of the sample surface, variations in microscopic electronic properties, and a potential distribution.
Recently, there have been proposed scanning tunneling spectroscopy (STS) for obtaining an image representing a variation in electronic properties on the surface of a sample, on the basis of a tunnel current, and scanning tunneling potentiometry (STP) for finding a potential distribution on the surface of a sample on the basis of a tunnel current.
An example of STP is disclosed in "Scanning tunneling potentiometry", Appl. Phys. Lett. 48(8), 24 Feb. 1986, pp. 514-516. An example of STP using a time sharing method is disclosed in "Direct Measurement of Potential Steps at Grain Boundaries in the Presence of Current Low", Phys. Rev. Lett. Vol. 60, No. 15, 11 Apr. 1988, pp. 1546-1549.
STP is employed to determine a potential distribution on the surface (xy-plane) of a sample. On the basis of the determined potential distribution, a potential gradient on the xy-plane is calculated. The potential gradient reflects local resistance of the sample or the mobility of charge, but does not reflect the motion of charge in the z-direction. Since no data is obtained with respect to the local electronic state representing the properties of the sample, the properties of the sample cannot be identified locally. In addition, no data is obtained with respect to the charge energy state which contributes to the transmission of electric current.
On the other hand, an example of STS is disclosed in "Surface Electronic Structure of Si (111)-(7.times.7) Resolved in Real Space", Phys. Rev. Lett. Vol. 56, No. 18, 5 May 1986, pp. 1972-1975. This document teaches current-imaging tunneling spectroscopy (CITS) wherein an electronic density distribution is found from the dependency of tunnel current I.sub.T upon bias current U.sub.B.
According to CITS, if a potential gradient is given to a sample, the dependency of tunnel current upon bias current is adversely affected locally by potential, and the original point for bias voltage scanning is displaced. As a result, absolute evaluation of the electronic state of the sample, using a zero level of bias voltage as a reference point, cannot be achieved. In addition, local potential of the sample cannot be identified.